The pharmacologic effects of intravenous melatonin on hypothalamic-adenohypophyseal function were studied in male rhesus monkeys (n = 10) and compared to control animals (n = 9–13). Basal and arginine- or L-dopa-stimulated values of growth hormone in melatonin-treated animals were similar to those of control primates. Insulin-stimulated growth hormone secretion was slightly decreased. Melatonin did not affect basal or thyroid stimulating hormone releasing hormone (TRH)-stimulated values of thyroid stimulating hormone (TSH) or prolactin. No effects were seen on basal and gonadotropin releasing hormone (GnRH)-stimulated luteinizing hormone (LH) or follicle stimulating hormone (FSH) secretio